1. Field of the Invention
The invention relates to a circuit arrangement for demodulating a video signal (IF signal) which is frequency-modulated on an intermediate-frequency carrier oscillation, comprising
a first demodulation stage formed as a frequency or phase demodulator for multiplicatively combining the IF signal with a carrier oscillation of a controllable oscillator and for supplying a first output signal formed from this combination, PA1 a second demodulation stage formed as a frequency or phase demodulator for multiplicatively combining the IF signal with the carrier oscillation phase-shifted by a quarter period of the intermediate-frequency carrier oscillation, and for supplying a second output signal formed from this combination, PA1 a first filter stage for low-pass filtering the first output signal and for supplying a third output signal, PA1 a multiplier stage for multiplicatively combining the second and the third output signals to derive a fourth output signal, PA1 a superposition stage for forming a seventh output signal by additively combining a fifth and a sixth output signal derived from the second and the fourth output signals, and PA1 a second filter stage for deriving a control signal from the seventh output signal for controlling the frequency and/or phase of the controllable oscillator. PA1 a first coefficient member for deriving the fifth output signal from the second output signal, and PA1 a second coefficient member for deriving the sixth output signal from the fourth output signal, PA1 the signal applied to each coefficient member being multiplied by a predeterminable coefficient for forming the signal derived therefrom and the coefficient of the first coefficient member being chosen to be larger than that of the second coefficient member.
2. Description of the Related Art
A video demodulator comprising a FPLL is disclosed in the article "An Advanced 5 V Vif-/SiF PLL for Signal Detection in TV Sets and VTRs" published in IEEE Trans. Cons. 1991, vol. 37, no. 4. This video demodulator comprises an oscillator (VCO) from which two 90.degree. phase-shifted oscillations are applied to a quadrature detector and to an in-phase detector via a frequency divider having a division factor of 2. Moreover, an intermediate frequency video signal is applied to these detectors. An output signal is applied as a control value from the quadrature detector to the oscillator via a current converter and a loop filter. The oscillator, the quadrature detector and the loop filter constitute a phase-locked loop. To reduce the pull-in time of this phase-locked loop during search tuning, the known arrangement also comprises a FLL which, in addition to the in-phase detector, comprises a low-pass filter and a multiplier. An output signal from the in-phase detector is applied to the multiplier via the low-pass filter, while a second input signal of the multiplier is constituted by the output signal of the quadrature detector. An output signal of the multiplier is superimposed on the current applied from the quadrature detector to the loop filter.
In this circuit arrangement, the FLL generates a current which is dependent on the frequency difference in the pull-in mode and is proportional to the phase difference in the locked mode. On the other hand, the phase-locked loop supplies a small direct current during the pull-in mode and its main current in the locked mode for controlling the phase. In the circuit arrangement described above, the currents of the FLL and the phase-locked loop are added, applied to the loop filter and subsequently applied as a control signal to the oscillator. The pull-in time is reduced by virtue of a wide pull-in range of the FLL.
An IF video signal processing circuit including a phase control circuit is known from DE-PS 40 21 912, corresponding to U.S. Pat. No. 5,105,273. This circuit comprises a synchronous demodulator which receives an intermediate-frequency video signal (IFV signal) and an output signal from a voltage-controlled oscillator and supplies a video demodulation output signal. In addition to the voltage-controlled oscillator, the phase control circuit comprises a phase control member which causes the phase of the output signal of the oscillator to lead by 90.degree., a phase detector which compares the phase between the output signal of the phase control member and the IFV signal, and a low-pass filter which filters the output signal of the phase detector and applies it to the control input of the voltage-controlled oscillator. In the locked state of the phase control circuit, the frequency and phase of the output signal of the voltage-controlled oscillator is rendered equal to the normal video carrier frequency of the IFV signal and applied to the synchronous demodulator. This demodulator demodulates the IFV signal synchronously on the basis of this signal while forming the video demodulation output signal.
In the circuit arrangement known from DE-PS 40 21 912, the operation of the phase detector is not only used for evaluating the IFV signal level but also is switched from PLL operation to locked operation on the basis of the video demodulation output signal. To this end, the phase detector has at least two further inputs, a first input of which receives a signal strength detection signal derived from the strength of the IFV signal, and a second input receives a modulation degree detection signal derived from the video demodulation output signal, while the video demodulation output signal is also applied to the phase detector.
In PLL operation, the phase detector compares the phase of the IFV signal and the output signal of the phase control member and supplies a phase detection signal. In the locked state, the phase detector locks in and supplies the phase detection signal supplied during the last PLL operation. To this end, the phase detector uses a capacitor which is arranged in the low-pass filter. In this known circuit arrangement, the PLL operation is switched on during the blanking periods of the video signal during which the phase detector is activated by the video demodulation output signal. In this state, the low-pass filter has a small time constant permitting fast reversals of the charge of the relevant capacitor. During the picture contents of the video demodulation output signal, the phase detector and the low-pass filter are, however, subjected to locked operation in that the phase detector is deactivated and the low-pass filter is switched to a large time constant. In this state of the low-pass filter, the relevant capacitor can reverse its charge only very slowly and the voltage value stored in this capacitor is therefore retained.
It is also known, from the discourse no. 23 by O. Hermsdorfer, "Der Synchrongleichrichter im Nyquist Messdemodulator--Aufbereitung des Schalttragers und Auswirkungen von Phasenstorhub auf das demodulierte Signal--" held at the FKTG conference from 4 to 7 Oct. 1976, to use a phase-locked loop in an in-phase demodulator for generating a carrier for demodulating an IF picture signal. In this loop, a signal from an oscillator (VCO) is compared in a phase discriminator with a signal which is obtained from the IF picture signal via a bandpass filter and a limiter so as to obtain a control voltage for the oscillator. Due to asymmetries in the sideband spectrum of the IF picture signal, an amplitude modulation of the picture carrier may be converted to an unwanted phase modulation during limitation and rectified in the phase discriminator of the phase-locked loop. This may lead to a phase modulation of the oscillator and hence to disturbances of the video output signal which is demodulated in the in-phase demodulator with the carrier from the oscillator. To avoid this, the voltage supplied by the phase discriminator is evaluated via a sample-and-hold circuit only at the instants when there is no picture modulation, for example at the instant of the backporch, and thus a substantially complete independence of the picture content is achieved.
In circuit arrangements for receiving and processing video signals of different transmission standards, using a synchronous demodulation of the type utilized in the circuit arrangements described hereinbefore, disturbances in the demodulated sync signals occur upon overmodulation of the television transmitters in the transmission standard "SECAM-L" used in France. This is caused by the positive modulation used in "SECAM-L" with a residual carrier for the synchronous signals. Practice has proved that even an inverted phase position of the demodulated signal may occur in the demodulation by means of the known circuit arrangements. In any case, strong distortions, particularly of the vertical sync pulses, result in demodulation arrangements in which the carriers are processed by means of phase-locked loops. As a result of such disturbances, the requirements to be imposed on the quality of the processed video signals and/or sync signals can no longer be maintained by the known circuit arrangements during operation.